The present invention relates, in general, to integrated circuits and, more particularly, to integrated circuits having mixer circuits for providing image rejection.
In electronic systems such as cellular or wireless telephones, a phase and frequency relationship exists between an incoming reference signal and a signal generated by a Local Oscillator (LO). Typically, mixers in a heterodyne receiver translate a Radio Frequency (RF) signal to a lower Intermediate Frequency (IF) signal. The mixers multiply the received RF signal with a Local Oscillator (LO) signal to generate the sum and difference frequencies between the RF and LO signals. A filter circuit is used to select the difference frequency, i.e., a mixed down signal, for recovering the modulation information that is contained in the translated RF signal.
Unwanted images having frequencies either higher or lower than the LO signal may also be present at the input of the receiver. The unwanted images are also translated in frequency by the mixers. It is desired that the signals at the outputs of the mixers be in-phase while the unwanted images be one hundred and eighty degrees out-of-phase and equal in amplitude. By summing the signals at the outputs of the mixers, the unwanted images being out-of-phase and equal in amplitude would completely cancel, while the desired signals would add to each other.
However, it is difficult to maintain equivalent signal attenuation through the mixers and linearity in phase shifters capable of generating a phase relationship of one hundred and eighty degrees between the unwanted images. Voltage and temperature changes in a heterodyne receiver cause signal distortions in phase shifters operating over a wide frequency range.
Accordingly, it would be advantageous to have a method and a circuit for frequency translating an incoming reference signal that cause unwanted images to be rejected.